The present invention generally relates to a technique for electrically blasting percussion powders of detonating primers, and more particularly to a method and an apparatus for blasting electrically a number of detonating primers electromagnetically coupled with a bus wire via magnetic cores by supplying a high frequency electric current to the bus wire.
FIG. 1 illustrates a known blasting apparatus by means of which a plurality of detonating primers electromagnetically coupled with a bus wire serially are electrically exploded. Such a known apparatus has been described in, for instance Japanese Patent Application Laid-open Publication No. 22,608/74. The apparatus comprises an oscillator unit 1 including a voltage supply source 2 such as a battery, a capacitor 3 charged by the battery 2, a trigger circuit 4 and a silicon controlled rectifier 5 which can be switched on by the trigger circuit 4. When the trigger circuit 4 is actuated, the oscillator unit 1 produces a pulsatory voltage having a large amplitude across outputs 6 and 7 to which a loop-like bus wire 8 is connected. Therefore a pulsatory current flows through the loop-like bus wire 8. To the bus wire 8 are electromagnetically coupled a number of transformer units 9 each having a magnetic core 10 and a secondary winding 11 wound on the core. Through the cores 10 the bus wire 8 is passed as a primary winding. To both ends of the secondary windings 11 are connected leg wires 12 and 13 of a number of detonating primers 14 which are coupled with blasting explosives 15. When the pulsatory current supplied from the oscillator unit 1 flows through the bus wire 8, pulsatory currents are electromagnetically induced in the leg wires 12 and 13 of the respective detonating primers 14, and thus the detonating primers 14 are exploded primarily. Then the explosives 15 coupled with the detonating primers 14 are blasted secondarily.
In the above explained known blasting method, since the current flowing through the bus wire 8, i.e. the primary winding of the transformer unit 9, has several hundred hertzs, the magnetic core 10 is liable to be large in size and heavy in weight. Therefore, it is difficult or at least cumbersome to carry and handle such large and heavy magnetic cores. In this connection, it should be noted that the number of detonating primers 14 to be coupled with the same and single bus wire 8 often amounts to several tens to several hundreds.
Moreover, in the known method, it is necessary to connect the leg wires 12 and 13 of respective detonating primer 14 to the secondary winding 11 which has been previously wound on the magnetic core 10. This requires very cumbersome operation of a user.
Furthermore, in the known method, the bus wire 8 should be passed through the ring-shaped magnetic cores 10 at places at which the explosion is to be effected. The operation for passing the bus wire through a number of cores is apparently very cumbersome. Particularly, when the number of magnetic cores 10 which have been passed through the bus wire 8 is increased, the bus wire is liable to become entangled with the magnetic cores 10. In order to avoid this, it is necessary to pull or draw the bus wire 8 rather strongly. However, when the bus wire 8 is pulled strongly, the leg wires 12, 13 of detonating primer 14 are subjected to a strong tensile force and unexpected shock might be applied to the detonating primer. This is very undesirable in the view point of safety. Moreover, in an extreme case the detonating primer 14 might be pulled out of the blasting explosive 15.
In the known method, the bus wire 8 forms a loop. Now it is assumed that the bus wire 8 has a diameter a and the loop has a diameter b as schematically illustrated in FIG. 2. Then an impedance Z of the loop formed by the bus wire 8 is expressed by the following equation, where f is a frequency of the current flowing through the bus wire 8, and N is the number of turns. EQU Z=2.pi.f.multidot.4.pi.bN.sup.2 (log 8b/a-2).multidot.10.sup.-7
From this equation, it is apparent that when an inductance of the loop represented by 4.pi.bN.sup.2 (log 8b/a-2)10.sup.-7 is changed, the impedance Z is also varied. The inductance of the loop will be changed in accordance with the configuration of the loop and the loop configuration will be varied in accordance with the fact how to lead or extend the bus wire 8. Further, in case of effecting the explosion under the sea, the loop of the bus wire will be changed in accordance with conditions of environment. Therefore, in the known method, the frequency of the output of the oscillator unit 1 has to be adjusted in accordance with the variation of the impedance of the bus wire 8 forming the loop, and therefore it is difficult to attain the stable operation under various conditions.